Abstract: To obtain more in-depth understanding on the physics of underexpanded excited jets as well as the flow response of the jets to external excitation at different nozzle pressure ratios （NPR）， three-dimensional large eddy simulation （LES） of underexpanded steady and excited jets are carried out at two different nozzle pressure ratios （NPR） of 5.60 and 9.34. The forcing frequencies are the inherent symmetric mode frequency in the steady jets of 14.569kHz， and the excited jets are implemented by imposing the sinusoidal disturbance to the static pressure at the nozzle entrance. The results indicate that the external excitations affect the acoustic patterns， reduce the region of the jet potential core， decrease the number of near-field shock cells， and have a remarkable impact on the mixing between the injected gas and the surroundings. Meanwhile， the dominant frequencies of the excited jets turn into the exciting frequency used and its high-order harmonics， and the dominant mode all switches to axisymmetric mode. In particular， the dominant mode of the steady jet at NPR=9.34 is same as the form of the external excitation， which results in intense flow resonance between the steady jet and the axisymmetric excitation. As a result， forcing the jet at NPR=9.34 provides more decrease in the length of jet potential core and the larger amplitude in pressure fluctuations， and the external excitations have a more effective influence on the jet mixing.

Key words: Underexpanded jets;Nozzle pressure ratio;Characteristic frequency excitations;Large eddy simulation;Flow response

摘要： 为了揭示欠膨胀激励射流的流动机理，以及考察不同喷压比下射流对相同激励的流动响应，采用大涡模拟方法，对喷压比NPR=5.60和9.34的欠膨胀定常射流和激励射流进行了三维数值计算。激励频率为定常射流中固有的轴对称频率=14.569kHz，激励形式为在射流喷管入口处施以正弦压力扰动。结果表明，特征频率激励影响射流的声场特征，缩小射流核心区的范围，减少射流近场的激波胞格数目，并影响射流气体与环境空气的混合。同时，激励射流的特征频率转变为激励频率及其高阶倍频，激励射流的主不稳定模态均为轴对称模态。其中，NPR=9.34的欠膨胀射流的主不稳定模态和外加压力扰动的形式相一致，射流与外加激励发生了更加剧烈的流动耦合和响应。这使得在NPR=9.34时，射流核心区长度减小得更多，压力脉动的振幅更大，激励对射流混合的增强作用更加明显。

关键词: 欠膨胀射流;喷压比;特征频率激励;大涡模拟;流动响应